LTM4633 datasheet(21/32 Pages) LINER

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LTM4633

4633f

For more information www.linear.com/LTM4633

applicaTions inForMaTion

If we take this equation an differentiate it with respect

temperature T, then:

dVD

= –

VGO – VD

This dVD/dT change as a function of temperature is the

typical ~–2.0mV/°C. This equation is simplified for the

first order derivation.

Solving for T, T = –(VG0–VD)/dVDprovidethetemperature.

1st Example: Figure 4 for 27°C, or 300°C Kelvin the diode

voltage is 0.598V, thus, 300°C = –(1200mV – 598mV)/

–2.0 mV/°C)

2nd Example: Figure 4 for 75°C, or 350°C Kelvin the diode

voltage is 0.50V, thus, 350°C = –(1200mV – 500mV)/

–2.0mV/°C)

Converting the Kelvin scale to Celsius is simply taking the

Kelvin temp and subtracting –273°C Kelvin from it.

A typical forward voltage is measured and placed in the

electrical characteristics section of the data sheet, and

Figure 4 is the plot of this forward voltage. Measure this

forward voltage at 27°C to establish a reference point.

Then use the above expression while measuring the

forward voltage over temperature will provide a general

temperature monitor.

The diode connected PNP transistor at the TEMP pins can

beusedtomonitortheinternaltemperatureoftheLTM4633.

A general temperature monitor can be implemented by

connecting a resistor between TEMP and VIN to set the

current to 100µA, and then monitoring the diode voltage

drop with temperature. See Figure 19 for an example.

Safety Considerations

The LTM4633 module does not provide galvanic isolation

from VIN to any of the three VOUTs. There is no internal

fuse. If required, a slow blow fuse with a rating higher

than the maximum input current can be used to protect

the unit in case of a catastrophic failure. An inline circuit

breaker function can also be used instead of a fuse.

The fuse or circuit breaker should be selected to limit the

current to the regulator during overvoltage in case of an

internal top MOSFET fault. If the internal top MOSFET fails,

then turning it off will not resolve the overvoltage, thus

the internal bottom MOSFET will turn on indefinitely trying

to protect the load. Under this fault condition, the input

voltage will source very large currents to ground through

the failed internal top MOSFET and enabled internal bot-

tom MOSFET. This can cause excessive heat and board

damage depending on how much power the input voltage

can deliver to this system. A fuse or circuit breaker can

be used as a secondary fault protector in this situation.

Layout Checklist/Example

The high integration of LTM4633 makes the PCB board

layoutverysimpleandeasy.However,tooptimizeitselectri-

cal and thermal performance, some layout considerations

are still necessary.

Use large PCB copper areas for high current paths, in-

cluding VIN, GND, VOUT1, VOUT2, and VOUT3. It helps to

minimize the PCB conduction loss and thermal stress.

Place high frequency ceramic input and output capacitors

next to the VIN, GND and the VOUT pins to minimize high

frequency noise.

Place a dedicated power ground layer underneath the unit.

To minimize the via conduction loss and reduce module

thermal stress, use multiple vias for interconnection be-

tween top layer and other power layers.

Donotputviasdirectlyonthepads,unlesstheyarecapped

or plated over. Use a separated SGND ground copper area

for components connected to signal pins. Connect the

SGND to GND underneath the unit. Bring out test points

on the signal pins for monitoring. Figure 6 gives a good

example of the recommended layout.

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